Steam hydrocarbon reforming is a method of producing hydrogen and other useful products from hydrocarbon containing streams such as natural gas. Products are generated using a large furnace, referred to as a steam hydrocarbon reformer where steam is reacted with the hydrocarbon containing stream at high temperature in the presence of a catalyst. For example, a steam methane reformer may be used to convert methane (CH4) into hydrogen gas (H2) and carbon monoxide (CO). Other systems are then used to further convert and purify the products of the steam hydrocarbon reformer.
A steam hydrocarbon reformer generally includes an industrial furnace heated by many burners, with the number of burners determined by the size and shape of the furnace. For example, a large steam hydrocarbon reformer furnace can reach 20 m×20 m×14 m in size, with hundreds of burners mounted on the inside walls. Inside the furnace, there are also many tubes filled with catalyst running in parallel from the top to the bottom of the furnace. Inside the tubes, a hydrocarbon stream is reacted with steam in the presence of a metal (typically nickel (Ni)) containing catalyst in order to produce hydrogen gas and CO. Outside the tubes, a fuel stream is burned with air through the burners to provide the heat needed for the reforming reaction taking place inside of the tubes. The tube walls separate the reactant stream from the fuel stream.
A steam hydrocarbon reformer usually consumes large amounts of fuel during its operation. The performance of the reforming process, including the production yield and energy consumption, is affected by a number of factors, for example the flow rates of the fuel through the burners, the ratio of steam to methane in the hydrocarbon feed input stream, the flow rate of the hydrocarbon feed input stream, the temperature profile of the furnace, and the type/amount/quality of the catalyst. Typically, the purity, flow rate and/or temperature of the reformer product (among other things) are monitored and used to adjust some of the variables mentioned in an effort to achieve a desired production rate and yield of the output stream. For example, an automatic control system may adjust the ratio of steam to methane, or adjust the fuel and/or air flow rates to the burners in response to a scheduled plant rate change or observed changes in temperature, purity, product yield, etc.
However, current control systems used for the steam hydrocarbon reformer furnaces are relatively simple and may only include proportional-integral-derivative (PID) or ratio control loops to control the steam hydrocarbon reformer outlet temperature and steam to carbon ratio, etc. Although these controllers usually achieve their control objectives relatively well, they do not typically result in an optimal performance of the whole furnace or smooth transitions under the conditions of plant rate changes or large disturbances. For example, by using the simple lead-lag and ratio control mechanism to maintain the desired steam to carbon ratio, certain disturbances can be introduced to the furnace operation if the lead-lag time is not properly selected for each operating condition. Also, controlling only the outlet temperature (i.e., the temperature of the mixture of products from all the tubes of the reformer) does not always result in a smooth temperature distribution across the furnace, which may lead to more energy consumption or hot spots in certain places.
Obviously, running and maintaining a steam hydrocarbon reforming furnace can be expensive, and an important objective for the operation of such a furnace is to reduce the capital and operational costs.